Animation Node-start of effect
Scale and Bias BAN, Blender Animation Nodes


We see your your nodes depicted in your question. Merci Mille Fois.
Two different approaches 


Custom Property FCurve referenced in BAN
Only BAN Math Nodes
You need only one approach for results.  Two for your education.



Custom Property FCurve Approach



BAN Nodes focusing on Custom Property FCurves. Image Above.



Bias and Scale in your Custom Property Graph.  Image Above.  The intended units are number of characters. Example. [0, 20].  Not a percentage. (Approximate.  Please Improve)


Method 1. Using the Custom Property FCurve.  Curve that you would see in Graph Editor.  This Custom Property Curve is introduced below. 


The Yellow Colored Nodes show the essentials. We take the zeroth [0] curve for the text object. We ask for its evaluation a number, which in my case is a range [0, 8] meaning the length of a string, not a percentage.  Debug Nodes show the numeric value and Data Path or less formally the Custom Property Name.  The name is displayed to verify correctness. Debug Name matches graph editor name which is good.
Debug Nodes are for the BAN writer.  They do not affect render.  Delete is desired.
You will need to take adjustment for your particular context.  Some cleanup work on my part is needed.  



BAN Math Nodes Only Approach



Gain and Bias with the Animate Float Node.  Please improve.  Approximately 2 nodes.  Attach in similar fashion to previous Trim Node.  Units in characters. You can judge for yourself if you like the absence of keyframe.  Image above.


Trim text node has start value zero.  Lets assume this is correct or a good start.  Possible improvement can come later. 
Lets assume text with trim node value [Start End] (0,0) is the empty string
Trim text node needs a new [End] value. Now add math nodes to create the formula indicated below.
Add a Math Subtract Node, Maximum Node, and Minimum Node. Connect them correctly.  Show your new nodes a soon as possible. Perfect or Imperfect.
Its better for you to show your nodes for your education.  You have already demonstrated your epic knowledge of Custom Property for an object.  I like approach one F-Curve better at this juncture.  So Math only nodes are a optional excercise for the reader.
In the discussion below you can substitute your number 120 for 10 below and make adjustments
An Object can have Custom Properties such as a number with Keyframe values. Keyframe creation implies a curve viewable in the graph editor.  Frame 10 may have value 0.00 and Frame 80 can have value 1.00  Blender can conveniently interpolate for you.  Blender has created a curve in the graph editor for you.  You have the full visual power of the graph editor at your command.




Custom property with Keyframes. Above image.


You might consider setting a percentage related to the current frame.
Using Math Functions. Example. Minimum(Maximum(Frame - 10,0), 80)/(80) produces ....  


Value 0.0 in the frame range [-infinity , 10]
Linear ramp values 0.0 to 1.0 in frame range [10, 80]
Value of 1.0 for frame range [80, infinity]
Scale and Bias (Shift).  Offset ramp by 10. Ramp to 80 with division scaling (/)
These values should be consistent with the graph above
BAN Blender Animation Nodes may have an equivalent visual graph or other useful nodes for this purpose
[a,b] denotes a range from frame a to frame b, requiring a &lt;= b
More abstractly, Minimum(Maximum(Frame - offset,0), end)/(end)

These values can for example determine the range of characters used for display 
Please forgive the lack of mathematical exactness and formatting above. Above is pseudo code.  You can find the exact function names such as Min.
Old Statement. Seeing your nodes would be an improvement.  Lots of details missing.  Lets forge ahead.